The development of the semiconductor technology has substantially been following the Moore's Law for over 40 years, and the main approach for improving the speed of the metal-oxide-semiconductor field effect transistors (MOSFET) and reducing the production cost is scalingdown of the geometric dimension of the device. With the development of the integrated circuit and scalingdown of the device size, some problems which can not be ignored have arisen, for example, increasing of the longitudinal electric field of a channel, the roughness of the interface between the gate oxidation layer which becomes increasingly thinner and the silicon substrate becoming worse, and increasing of the scattering of the channel impurities, etc., all of which deteriorate the mobility of the charge carriers. In order to provide relatively large driving current to ensure relatively high speed of a device, reduction of driving current caused by the deterioration of the mobility is a problem difficult to handle and urgently to be solved.
Compared with silicon material, the mobility of holes in germanium material under a low electric field is 4 times as large as that in the silicon material, and the mobility of electrons in the germanium material is 3 times as large as that in the silicon material. Therefore, as a novel material for channel, the germanium material becomes one of the promising candidates in high speed MOSFET device due to its higher and more symmetrical mobility of charge carriers, which is also a hot topic of current research.
Further, in addition to applying novel channel material (such as germanium) to increase the mobility of charge carriers, strain technology is also a good choice. The mobility of charge carriers in the channel may be increased so as to enhance the driving current and improve the performance of a device by applying stress in a channel of the device and changing the band structure of the semiconductor through methods such as altering the material, structure or process of the device, etc. Wherein, the process induced strain method due to its convenient and effective characteristics becomes a method for introducing strain which is currently widely used in mass production in the industry. With respect to the NMOS device, the strain technology may introduce a uniaxial tensile stress in the direction parallel to the channel plane and also may simultaneously introduce a compressive stress in the direction perpendicular to the channel plane, so as to increase the mobility of the electrons in the channel.
To sum up, how to increase the mobility of charge carriers in the channel is one of the difficulties urgently to be solved in conventional ultra large scale integrated circuit fabrication technology.